1. Field of the Invention
The present invention relates to an image-formation optical system in which a light beam is optically converged at an information medium such as an optical medium or a magneto-optical medium like an optical disk or an optical card, an optical head apparatus in which information is written in the information medium with the image-formation optical system and the written information is read out or erased, and an optical information apparatus in which a positional relation between the information medium and the optical head apparatus is precisely adjusted. Also, the present invention relates to an information writing and reading method for optically writing information in the information medium with the image-formation optical system and reading the information with the optical head apparatus.
2. Description of the Related Art
An optical memory technique has been put to practical use to manufacture an optical disk in which a pit pattern indicating information is formed. The optical disk is utilized as a high density and large capacity of information medium. For example, the optical disk is utilized for a digital audio disk, a video disk, a document file disk, and a data file disk. To store information in the optical disk and to read the information from the optical disk, a light beam radiated from a light source is minutely narrowed in diameter in an image-formation optical system, and the light beam minutely narrowed is radiated to the optical disk through the image-formation optical system. Therefore, the light beam is required to be reliably controlled in the image-formation optical system with high accuracy.
The image-formation optical system is utilized for an optical head apparatus in which a detector is additionally provided to detect intensity of the light beam reflected by the optical disk. Also, the optical head apparatus is utilized for an optical information apparatus in which a control section is additionally provided to adjust a positional relation between the optical disk and the optical head apparatus. Fundamental functions of the optical information apparatus are classified into converging performance for minutely narrowing a light beam to form a diffraction-limited micro-spot of the light beam radiated on an optical disk, focus control in a focus servo system, tracking control in a tracking serve system, and detection of pit signals (or information signals) obtained by radiating the light beam on a pit pattern of the optical disk.
The image-formation optical system is composed of a light source for radiating a light beam, and a group of lenses including an objective lens for converging the light beam at an optical disk and directing the light beam reflected by the optical disk to an information signal detecting system. The information signal detecting system is provided with optical elements for dividing the light beam into signal light beams indicating various signals such as an information signal, a photo detector for detecting intensities of the signal light beams, and an actuating unit for moving the objective lens of the image-formation optical system. The optical head apparatus is composed of the image-formation optical system and the information signal detecting system. The optical information apparatus is composed of the optical head apparatus and a control section for controlling the position of the objective lens of the image-formation optical system under control of a focus servo system and a tracking servo system. A gas laser is initially utilized as the light source, and a semiconductor laser has been recently utilized as the light source because the semiconductor laser can be manufactured at a moderate cost in a small size.
However, in cases where the semiconductor laser is utilized as the light source, a driving current supplied to the semiconductor laser changes each time a writing operation and a reading operation are exchanged for each other. Therefore, a refractive index of semiconductor laser material changes in dependence on the driving current. As a result, a wavelength of the light beam changes each time the writing operation and the reading operation are exchanged for each other. In this case, a refractive index of the objective lens for the light beam changes in dependence on the change of the wavelength of the light beam. Therefore, a light spot of the light beam converged at the optical disk is in a defocus condition until focus control in a focus servo system follows up the change of the wavelength of the light beam. That is, there is a drawback that the exchange of the reading and writing operations cannot be quickly performed.
To reduce the change of the refractive index of the objective lens, a first trial in which the objective lens is made of material having a low wavelength-dispersion characteristic has been proposed. However, because the material having the low wavelength-dispersion characteristic has a low refractive index, a curvature of the objective lens is required to be enlarged. Therefore, it is difficult to make the objective lens having a large numerical aperture (NA) and the low wavelength-dispersion characteristic. Also, a second trial in which a combination lens formed by combining a plurality of lenses having various wavelength-dispersion characteristics is utilized for the image-formation optical system has been proposed.
2.1. First Previously Proposed Art
FIG. 1 is a constitutional view of a conventional optical head apparatus.
As shown in FIG. 1, a light beam B1 linearly polarized is radiated from a semiconductor laser 11 in a conventional optical head apparatus 10. The light beam B1 is collimated by a combination lens 12, and the cross section of the light beam B1 is reshaped in circular shape by a wedge-like prism 13. Thereafter, the light beam B1 transmits through a beam splitter 14 and is converged by an objective lens 15 at an information medium in an outgoing light path. In this case, the position of the objective lens 15 is adjusted with an actuating unit 17 to focus the objective lens 15 on the information medium 16. Therefore, a light spot Ls is formed on the information medium 16. The light beam B1 reflected by the information medium 16 transmits through the objective lens 15 in an incoming light path, and a major part of the light beam B1 is reflected by the beam splitter 14. Thereafter, the light beam B1 is converged by a collimator lens 18, and a wavefront of the light beam B1 is changed in a servo signal detecting unit 19 to obtain a focus error signal and a tracking error signal. Thereafter, the intensity of the light beam B1 is detected in a photo detector 20. Therefore, the focus and tracking error signals and an information signal is obtained by calculating the intensity of the light beam B1 detected, and the actuating unit 17 is moved in dependence on the focus and tracking error signals to adjust the position of the objective lens
In this case, to quickly move the objective lens 15, the objective lens 15 is required to be lightweight. Therefore, a combined lens composed of a plurality of refracting lenses cannot be utilized for the objective lens 15. As a result, chromatic aberration of the objective lens 15 necessarily exists because of the change of the refractive index in the objective lens 15 for the beam light B1 of which the wavelength changes each time the reading and writing operations are exchanged for each other. To reduce adverse influence of the chromatic aberration of the objective lens 15, chromatic aberration of the combination lens 12 is excessively corrected to cancel out the chromatic aberration of the objective lens 15. That is, a focal length of the objective lens 15 is lengthened as the wavelength of the light beam B1 becomes longer because of the increase of the driving current supplied to the semiconductor laser 11. In contrast, a focal length of the combination lens 12 is shortened as the wavelength of the light beam B1 becomes longer.
2.2. Second Previously Proposed Art
Next, an example of achromatization performed in a single lens is shown in FIG. 2. The lens utilized for the achromatization has not been applied to any optical head apparatus.
FIG. 2 is a cross-sectional view of a conventional achromatic lens in which a hologram lens is combined.
As shown in FIG. 2, a conventional achromatic lens 21 consists of a diffraction grating type of hologram lens 22 and a refracting lens 23. Where a symbol f.sub.Ho denotes a focal length of the hologram lens 22 for a light beam L.sub.B having a wavelength .lambda..sub.o and a symbol f.sub.H1 denotes a focal length of the hologram lens 22 for another light beam L.sub.B having a wavelength .lambda..sub.1, an equation (1) is satisfied. EQU f.sub.H1 =f.sub.Ho .times..lambda..sub.o /.lambda..sub.1 ( 1)
The focal length f.sub.H of the hologram lens 22 is shortened as the wavelength .lambda. of the light beam L.sub.B becomes longer. Also, where a symbol n(.lambda.) denotes a refractive index of the refracting lens 23 for the light beam L.sub.B having the wavelength .lambda. and a symbol f.sub.D (.lambda.) denotes a focal length of the refracting lens 23 for the light beam L.sub.B having the wavelength .lambda., an equation (2) is satisfied. EQU f.sub.D (.lambda..sub.1)=f.sub.D (.lambda..sub.o).times.(n(.lambda..sub.o)-1)/(n(.lambda..sub.1)-1) (2)
The focal length f.sub.D (.lambda.) of the refracting lens 23 is lengthened as the wavelength .lambda. of the light beam L.sub.B becomes longer. That is, the dependence of the focal length f.sub.D (.lambda.) on the wavelength .lambda. of the refracting lens 23 is opposite to that in the hologram lens 22. Therefore, a condition that a combination lens of the lenses 22, 23 functions as the achromatic lens 21 is formulated by an equation (3). ##EQU1##
Accordingly, because the dependence of the focal length f.sub.D (.lambda.) on the wavelength .lambda. of the refracting lens 23 is opposite to that in the hologram lens 22, the achromatic lens 21 can be formed by the combination of the lenses 22, 23. Therefore, curvature of the achromatic lens 21 can be small. Also, because the hologram lens 22 is a plane type of element, the lightweight achromatic lens 21 can be made in large scale manufacture. The conventional achromatic lens has been proposed in a first literature (D. Faklis and M. Morris, Photonics Spectra (1991), November p.205 & December p.131), a second literature (M. A. Gan et al., S.P.I.E. (1991), Vol.1507, p.116), and a third literature (P. Twardowski and P. Meirueis, S.P.I.E. (1991), Vol.1507, p.55).
Also, as is described in the first literature, the hologram lens 22 is manufactured according to a manufacturing method shown in FIGS. 3A to 3F.
FIGS. 3A to 3F are respectively a cross-sectional view showing a manufacturing method of the hologram lens 22.
As shown in FIG. 3A, a hologram substrate 24 is coated with a resist 25, and the resist 25 is covered with a first patterned photomask 26. Thereafter, the resist 25 is exposed to ultraviolet radiation to transfer a first pattern to the resist 25. After the photomask 26 is taken off, the resist 25 exposed is developed to pattern the resist 25 with the first pattern as shown in FIG. 3B. After development, the hologram substrate 24 exposed is etched with an etchant at a depth H1 in a first etching process, and the resist 25 is stripped as shown in FIG. 3C.
Thereafter, the hologram substrate 24 etched is again coated with a resist 27, and the resist 27 is covered with a second patterned photo mask 28 as shown in FIG. 3D. Thereafter, the resist 27 is exposed to ultraviolet radiation to transfer a second pattern to the resist 27. After the photomask 28 is taken off, the resist 27 exposed is developed to pattern the resist 27 with the second pattern as shown in FIG. 3E. After development, the hologram substrate 24 exposed is again etched with an etchant at a depth H2 in a second etching process, and the resist 27 is stripped as shown in FIG. 3F.
Accordingly, the hologram lens 22 of which the surface is blazed and formed in echelon shape as a multilevel hologram can be manufactured by repeating a lithography process and an etching process.
Also, another manufacturing method of a hologram lens has been proposed in a fourth literature (K. Goto et al., J.J.A.P. (1987), Vol. 26, Supplement 26-4).
FIG. 4 shows an original form of a hologram substrate patterned with a cutting tool.
As shown in FIG. 4, a hologram lens 29 can be manufactured with a cutting tool 30 of a super precision CNC lathe.
In the fourth literature, a combination lens of a spherical lens and the hologram lens 29 is applied to an optical head apparatus to reduce aberration such as off-axis aberration according to an aspherical lens effect. Therefore, chromatic aberration of the combination lens is not corrected. That is, when the wavelength of a light beam radiated from a semiconductor laser fluctuates, the focal point of the combination lens is moved.
2.3. Third Previously Proposed Art
A prior art laid open to public inspection under Provisional Publication No. 155514/91 (H3-155514) and under Provisional Publication No. 155515/91 (H3-155515) is cited as a third previously proposed art.
FIG. 5 is a cross-sectional view of a conventional optically converging system consisting of an objective lens and a chromatic aberration correction element.
As shown in FIG. 5, a conventional optically converging system 31 consists of an objective lens 32 focused on an information medium 33 and an chromatic aberration correction lens 34 arranged at a light source side. The objective lens 32 is allowed to be exchanged for a hologram lens. The chromatic aberration correction lens 34 is a combination lens of a positive lens 35 (or a convex lens) and a negative lens 36 (or a concave lens). Though the chromatic aberration correction lens 34 has no lens function, chromatic aberration of the objective lens 32 is corrected with the chromatic aberration correction lens 34 because a wavelength-dispersion coefficient of the positive lens 35 differs from that of the negative lens 36.
Therefore, the chromatic aberration correction lens 34 functions in the same manner as the combination lens 12.
2.4. Problems to be Solved by the Invention
In cases where a light beam radiated from a semiconductor laser is converged by a lens to form an image, the lens generally has astigmatic aberration because an astigmatic difference necessarily occurs in an active layer of the semiconductor laser. The reason that the astigmatic difference occurs in the active layer is described with reference to FIGS. 6 and 7.
As shown in FIG. 6, where a longitudinal direction of an active layer 37 in an end facet 11a of the semiconductor laser 11 is defined as a horizontal direction, an outgoing radiation point Pr.sub.H in the horizontal direction is positioned within the active layer 37 by a length .delta.. In contrast, another outgoing radiation point Pry in a vertical direction perpendicular to the horizontal direction is positioned at the end facet 11a. Therefore, the astigmatic difference occurs in the active layer 37 so that the astigmatic aberration occurs in the conventional optical head apparatus 10.
To remove the astigmatic aberration, a collimator lens 38 composed of the combination lens 12 and the wedge-like prism 13 in the first previously proposed art is moved in the outgoing light path direction. In detail, because an elliptic cross section of the light beam B1 is corrected in circular shape with the wedge-like prism 13, a focal length f.sub.cv of the collimator lens 38 in the vertical direction differ from another focal length f.sub.CH of the collimator lens 38 in the horizontal direction, as shown in FIGS. 7(a), 7(b). A light beam path in the vertical direction from the outgoing radiation point Pr.sub.v to the information medium 16 is shown in FIG. 7(a), and a light beam path in the horizontal direction from the outgoing radiation point Pr.sub.H to the information medium 16 is shown in FIG. 7(b). Symbols shown in FIGS. 7(a), 7(b) denote as follows.
f.sub.o : a focal length of the objective lens 15;
f.sub.c : a focal length of the combined lens 12, and a focal length of first collimator lens 44 in embodiments according to the present invention;
f.sub.cv : an equivalent focal length of the collimator lens 38 in the vertical direction (=f.sub.c);
f.sub.CH : an equivalent focal length of the collimator lens 38 in the horizontal direction (=f.sub.c .times..gamma.);
.gamma.: an elliptic beam correction coefficient in the wedge-like prism 13, .gamma.&gt;1;
.delta.: the astigmatic difference;
.delta..sub.v : a difference between a focal point Fc of the collimator lens 38 and an objective point (or the outgoing radiation point Pr.sub.v) in the vertical direction; .delta..sub.H : a difference between the focal point Fc of the collimator lens 38 and an objective point (or the outgoing radiation point Pr.sub.H) in the horizontal direction;
.epsilon..sub.v : a difference between a focal point Fo of the objective lens 15 and an image point Pi.sub.v in the vertical direction; and
.epsilon..sub.H : a difference between the focal point Fo of the objective lens 15 and an image point Pi.sub.H in the horizontal direction.
To set a relationship .epsilon..sub.v =.epsilon..sub.H, the collimator lens 38 is moved towards the objective lens 15 to increase the difference .delta..sub.v (.delta..sub.v &gt;0) so that the focal point Fc positioned at the end facet 11a of the semiconductor laser 11 is also moved towards the objective lens 15. Therefore, the image points Pi.sub.v, Pi.sub.H in the horizontal and vertical directions are moved towards the objective lens 15.
Because a relation in longitudinal magnification is satisfied, equations (4), (5) are obtained. ##EQU2##
To remove the astigmatic aberration occurring in an image which reflects on the information medium 16, the relationship .epsilon..sub.v =.epsilon..sub.H is required. Therefore, an equation (6) is obtained by use of the equations (4), (5). EQU .delta..sub.v .times..gamma..sup.2 =.delta..sub.H ( 6)
Therefore, in cases where the collimator lens 38 is moved towards the objective lens 15 on condition that the equation (6) is satisfied, the astigmatic aberration is removed.
However, even though the position of the collimator lens 38 is adjusted to satisfy the equation (6) in the reading operation in which intensity of the light beam B1 is low because the driving current supplied to the semiconductor laser 11 is low, the astigmatic aberration occurs in an image reflecting on the Information medium 16 in the writing operation in which intensity of the light beam B1 is high because the driving current supplied to the semiconductor laser 11 is high.
In detail, as the intensity of the light beam B1 is increased, the wavelength of the light beam B1 is lengthened. Therefore, the focal length f.sub.c of the collimator lens 38 is shortened because the chromatic aberration of the combination lens 12 is excessively corrected to cancel out the chromatic aberration of the objective lens 15. As a result, the difference .delta..sub.v is increased to maintain a summed value .delta..sub.v +f.sub.c of the difference .delta..sub.v and the focal length f.sub.c, and the difference .delta..sub.H is increased to maintain a summed value .delta..sub.H +f.sub.c of the difference .delta..sub.v and the focal length f.sub.c. Accordingly, a ratio .delta..sub.H /.delta..sub.v becomes smaller than a value .gamma..sup.2 in the writing operation. In addition, as the intensity of the light beam B1 is increased, the astigmatic difference .delta. is decreased, as is well known. Therefore, the ratio .delta..sub.H /.delta..sub.v is moreover decreased in the writing operation.
Accordingly, in cases where the reading operation and the writing operation are exchanged for each other in the conventional optical head apparatus 10, the collimator lens 38 cannot be moved on condition that the equation (6) is satisfied in the writing operation. Also, in cases where the position of the collimator lens 38 is adjusted to satisfy the equation (6) in the writing operation, the ratio .delta..sub.H /.delta..sub.v becomes larger than the value .gamma..sup.2. Therefore, the collimator lens 38 cannot be moved to satisfy the equation (6) in the reading operation. As a result, there is a drawback that the astigmatic aberration necessarily occurs on the information medium 16.
Next, various drawbacks in the conventional achromatic lens 21 of the second previously proposed art are described.
The conventional achromatic lens 21 has no chromatic aberration. However, in cases where the lens 21 is applied to an image-formed optical system or an optical head apparatus, many drawbacks occur as follows.
1. To apply the achromatic lens 21 to the optical head apparatus 10, the chromatic aberration of each of lenses utilized in the optical system is required to be corrected. Therefore, degree of freedom in design is considerably decreased. Specifically,. though the astigmatic difference .delta. varies in dependence on the change in the output intensity of the semiconductor laser 11, the lenses cannot be designed so as to compensate the variation of the astigmatic difference .delta.. Therefore, the astigmatic aberration necessarily occurs in the light spot Ls on the information medium 16, so that an information signal obtained in the apparatus 10 deteriorates.
2. To apply the achromatic lens 21 to the optical head apparatus 10, three hologram lenses 22 are required to be utilized in the collimator lenses 18, 38 and the objective lens 15 for the purpose of the correction of the chromatic aberration of the lenses 15, 18, 38. Therefore, many number of hologram lenses 22 are provided in the apparatus 10, so that the apparatus 10 cannot be manufactured at a moderate cost.
3. A diffraction efficiency of the hologram lens 22 varies in dependence on the wavelength of light transmitting through the hologram lens 22. Also, a design method for appropriately setting the diffraction efficiency in an optical head apparatus has never been proposed in the prior art. Therefore, in cases where the achromatic lens 21 is applied to the optical head apparatus, the diffraction efficiency of the hologram lens 22 becomes lowered in the reading operation. In this case, even though first-order diffraction light is mainly generated in the hologram lens 22 to obtain an information signal, other light such as zero-order diffraction light (or transmitting light), minus first-order diffraction light, and second-order diffraction light is undesirably generated in the hologram lens 22. The undesired other light becomes stray light so that the undesired other light functions as noise. Therefore, a signal-noise ratio (S/N ratio) considerably deteriorates.
Next, various drawbacks in the conventional optical system 31 of the third previously proposed art are described.
1. Because the chromatic aberration correction lens 34 is formed by combining the positive lens 35 and the negative lens 36, manufacturing costs such as material costs, production costs, combination costs including adjustment costs of the positive lens 35 and the negative lens 36, and attachment costs are required. Therefore, the conventional optical system 31 cannot be applied to an image-formation optical system or an optical head apparatus at a moderate cost.
2. Because the chromatic aberration correction lens 34 is formed by combining the positive lens 35 and the negative lens 36, the lens 34 becomes heavy and large. Therefore, in cases where the lens 34 is utilized in an image-formation optical system or an optical head apparatus, the system or the apparatus becomes heavy and large.
3. Because the chromatic aberration correction lens 34 is heavy and because the objective lens 32 is required to be slightly moved at high speed under control of a focus servo system and a tracking servo system, the lens 34 cannot be integrally formed with the objective lens 32 on condition that the objective lens 32 and the lens 34 are slightly moved at high speed. Therefore, a positional relation between the lenses 32, 34 changes when the objective lens 32 is moved. Therefore, even though the chromatic aberration of the objective lens 32 is always corrected by the chromatic aberration correction lens 34 regardless of the change in the positional relation, other aberration such as the astigmatic aberration are required to be independently corrected. In this case, degree of freedom in the design of lenses such as the objective lens 32 becomes low. As a result, aspherical lenses are required in an image-formation optical system or an optical head apparatus. Accordingly, the design and manufacturing of the system or the apparatus becomes difficult, and the system or the apparatus cannot be manufactured at a moderate cost.
4. The chromatic aberration correction lens 34 is heavy. Also, the objective lens 32 is slightly moved at high speed under control of a focus servo system and a tracking servo system. Therefore, the lens 34 cannot be integrally formed with the objective lens 32. In this case, a holding element for holding the lens 34 is required independently of another holding element for holding the objective lens 32. Accordingly, the conventional optical system 31 cannot be applied to an image-formation optical system or an optical head apparatus at a moderate cost, and the system or the apparatus becomes large.